1. Field of the Invention
This invention relates to crystal resonators for oscillators and stress sensitive transducers, and more particularly, to improved resonator mounting structures having low sensitivities to environmental effects.
2. Description of the Prior Art
Crystal oscillators with low sensitivity to temperature variations have been widely used as frequency standards. These oscillators, usually fabricated from quartz, may achieve their temperature insensitivity by careful selection of crystallographic axes, compensation through electronic oscillator components, compensation through additional resonators and utilization of combinations of vibrational modes, such as flexural and torsional modes, to achieve specific temperature related performance. Tradeoffs exist in temperature effects versus crystal impedance and other performance aspects as well as manufacturability and cost considerations. It is desirable to have an additional temperature related degree of freedom affecting crystal frequency. This additional flexibility in design is derived from the concept of thermally induced mechanical stress applied to a load sensitive crystal.
In contrast to crystal resonators used as transducers, crystal resonators used as frequency standards should be isolated from and unaffected by external forces. In general, crystal frequency standards have been designed with thin wire mountings attached to nodal points of vibration such that stress cannot be applied to the crystals.
A number of load sensitive crystals and transducers are known. In an unstressed condition, under constant environmental conditions, a crystal has a unique resonant frequency determined by its dimensions and material composition. The resonant frequency increases under tensile loading and decreases under compressive loading. The resonant frequency should thus be a true and accurate measure of the applied load.
Force sensitive crystals are described in U.S. Pat. No. 2,984,111 issued to Kritz and U.S. Pat. No. 3,093,760 issued to Tarasevich in which loads are applied to the crystals near the nodal points. Imprecise location of these nodal points results in energy transmission through the mounts, degrading the "Q", or quality factor, of the resonator with a consequential loss of accuracy. Also, forces and moments produced by the mounting structure can be transmitted to the resonator due to the imprecise nodal point location.
U.S. Pat. No. 3,470,400 issued to Weisbord describes a single beam force transducer with an integral mounting system which effectively decouples the beam vibrations from the mounting point through a spring and mass arrangement. This resonator is complex, relatively large, and difficult to manufacture.
A potentially small, simple and easy to manufacture device using photolithography is the closed end tuning fork described in U.S. Pat. No. 3,238,789 issued to Erdley. The Erdley device consists of two tines or bars vibrating 180 degrees out of phase such that the reactive forces and moments cancel.
Techniques for developing resonators with a low temperature coefficient using combinations of flexural and torsional modes of vibration were described by Momosaki et al in a paper presented in 1979 at the 33rd Annual Symposium on Frequency Control. This approach is both complex and restrictive. A new variable relating frequency to temperature can offer flexibility in design as well as improved performance.
One technique for temperature compensating crystal resonators is described in a paper presented in 1961 by Gerber et al at the Fifteenth Annual Symposium on Frequency Control. Gerber et al disclose mounting a relatively high frequency, shear mode crystal resonator on a base, and biasing the resonator against the base by a pair of temperature sensitive bimetalic bars. One of the bars is used for relatively low temperatures while the other bar is used for higher temperatures. While the Gerber et al approach may be satisfactory for temperature compensating shear mode crystals which are highly insensitive to external forces, it is not applicable to flexurally vibrating crystal resonators which are orders of magnitude more sensitive to external forces. In fact, if the teachings of Gerber et al were applied to flexurally vibrating resonators excessive forces would certainly be imposed on the resonators by the bi-metallic bars.